The invention relates to a needle-less injector having a housing comprising a chamber defined within said injector for containing liquid to be injected; a liquid outlet for said chamber positioned at the front end of the injector; a dispensing member in contact with the liquid in said chamber and movable in a first direction to reduce the volume of said chamber to cause the liquid contained therein to be expelled through said liquid outlet; an impacting member arranged to strike said dispensing member to cause movement thereof in said first direction; and drive means connected to the impacting member for actuating said injector, or permitting actuation thereof, in response to the application of a selected amount of axial pressure.
Such a needle-less injector is known from U.S. Pat. No. 5,480,381. This known needle-less injector has a liquid metering chamber with an outlet orifice, a piston slidable within the metering chamber, and a rod arranged to strike one end of the piston to force liquid through the outlet orifice. A cam follower is mounted at one end of the rod to ride on a cam that retracts the rod away from the piston. The injector is in two parts which are biassed away from each other. Actuation occurs when the two parts of the injector are sufficiently urged towards each other by the user to actuate the drive means in order to move the cam for release of the rod and thereby strike the piston and force liquid through the outlet orifice.
There are several problems attaching to this known injector. One problem is that of lacking accuracy regarding the placement of the cam follower on the highest position of the cam, which is the position of the cam follower when the injector is not used. Due to this construction unintended firing of the injector can occur, for instance due to shocks to which the injector is exposed. This construction gives also rise to occasional repeated firing of the injector when the apparatus is placed on the epidermis of the subject or object to be injected.
When the user urges the two parts of the injector towards each other, actuation occurs by moving a resilient strip to activate a micro switch. The problem associated with this device is that repeated actuation changes the characteristics of this resilient strip, causing a need for readjustment of the injector after a prolonged period of use. A further problem is that the known injector requires careful adjustment after assembly, which is hard and therefore relatively costly. Still another problem is that the accuracy of the amount of liquid be expelled from the chamber containing said liquid, is rather poor and very largely depending on the amount of counterpressure caused by the epidermis of the human, animal or plant on which the injector is placed.
The invention aims to reduce or set aside said problems and the needle-less injector according to the invention is therefore characterized in that the housing is essentially unitary and the chamber, the liquid outlet and the drive means are immobile with respect to the housing, and further that the injector comprises a pressure sensor in order to sense the axial pressure, which pressure sensor is connected to switching means within the housing which are connected to the drive means and a power source for said drive means, and which switching means is capable to establish contact between the drive means for actuating said injector and the power source when the pressure sensor is actuated by the selected amount of axial pressure.
By this construction, the complex two part housing with spring activated movable elements located in said housing as proposed in the state of the art is avoided. The construction according to the invention is simpler, easier to manufacture and to assemble, less costly and safer and more accurate in use.
Consistent with the prior art, the needle-less injector according to the invention has drive means which are connected to the impacting member via a cam and a cam follower for holding the said impacting member away from the said dispensing member against a biasing force, and for releasing the said impacting member to permit it to travel towards and impact against the said dispensing member. According to the invention. however, a preferred embodiment is characterized in that in the unloaded state of the pressure sensor, the cam follower is located near the beginning of the ramp provided by the cam, corresponding with a minimum volume of said chamber for containing the liquid to be injected. This provides more safety to the injector and immunity against shocks which would otherwise lead to premature firing of the injector. This also prevents the above-mentioned occasional repeated firing when the injector is placed on the epidermis of a human, animal or plant.
In a further preferred embodiment, the injector according to the invention is characterized in that in the loaded position of the pressure sensor, the switching means are enabled to establish contact between the power source and the drive means for actuating the injector so as to rotate the cam in order to have the cam follower travel from the beginning of the cam""s ramp to the cam""s highest position and beyond until it returns to the position near the beginning of the cam""s ramp, and correspondingly to effect moving the said impacting member away from the said dispensing member against the biasing force in order to increase the volume of said chamber for containing the liquid to be injected, and, when the cam follower travels beyond the cam""s highest position, to effect releasing the said impacting member to permit it to travel towards and impact against the said dispensing member to cause the liquid to be expelled from the chamber through the said liquid outlet, and that the switching means disable further contact between the power source and the drive means until at least the pressure sensor is brought in the unloaded state, and the cam follower has returned to the beginning of the cam""s ramp. This avoids the accuracy problems of the injector according to the state of the art which required an accurate positioning of the cam follower on the highest point of the cam just prior to firing when placed against the epidermis of the object to be injected.
In an alternative advantageous embodiment, the injector is provided with a trigger switch, which in its actuated state enables the switching means to establish contact between the power source and the drive means for actuating the injector so as to rotate the cam in order to have the cam follower travel from the beginning of the cam""s ramp to the cam""s highest position, and remain there by disabling the contact between the power source and the drive means, and correspondingly to effect moving the said impacting member away from the said dispensing member against the biasing force in order to increase the volume of said chamber for containing the liquid to be injected, and that said contact between the power source and the drive means is resumed and maintained, and consequently further rotation of the cam so as to cause the cam follower to travel beyond the cam""s highest position is effectuated by simultaneous actuation of the trigger switch and the pressure sensor, until the cam follower returns to the position near the beginning of the cam""s ramp so as to effect releasing the said impacting member to permit it to travel towards and impact against the said dispensing member to cause the liquid to be expelled from the chamber through the said liquid outlet, and that the switching means disable further contact between the power source and the drive means until at least the pressure sensor is brought in the unloaded state, and the cam follower has returned to the beginning of the cam""s ramp.
The pressure sensor can be electronic or a simple mechanical solution such as a rod extending from the housing; a reliable and preferred mechanical solution is however characterized in that the pressure sensor is a front portion which is movable with respect to the housing by the selected amount of axial pressure from an unloaded to a loaded position.
In a further preferred embodiment, the switching means includes an optical switch, part of which is attached to the front portion so as to establish whether the front portion is in the loaded or the unloaded position.